The present invention relates to sample and hold demodulator circuits and particularly, though not exclusively, to sample and hold demodulator circuits for use in automotive immobilizer applications.
It is well known that amplitude and/or phase demodulation can be simply achieved by using a sample and hold circuit. However, this technique requires accurate knowledge of the input signal""s phase shift relative to a reference clock.
Many system parameters can influence this phase shift, preventing use of fixed approximation techniques and necessitating actual measurement. The measured value must then be computed into an appropriate sampling time.
In applications such as automotive immobilizers, it is known to implement a sample and hold demodulator in a base station electronic control unit (ECU) for location in an automobile as a circuit partitioned in two parts: a front end receiver, and a calculator (typically a microcontroller). The receiver measures the phase and sends this measurement to the microcontroller via a bus. The microcontroller computes the corresponding sampling time and sends it to the receiver for beginning the demodulation.
This known implementation requires either that there is a dedicated microcontroller in the base station module, or that there are extra interfaces and wires between the base station and a remote microcontroller (e.g., in the main ECU elsewhere in the automobile).
Such an implementation therefore suffers increased cost and/or complexity.
It is an object of this invention to provide a sample and hold demodulator circuit in which the above disadvantages may be overcome or at least alleviated.
In accordance with an embodiment of the invention there is provided a sample and hold demodulator circuit for use in an automotive immobilizer to recover modulation information from a received modulated carrier signal (VRD). Sample and hold circuitry samples signals to recover the modulation information therein, and control circuitry is coupled to the sample and hold circuitry for controlling operation thereof. The control circuitry includes shift register circuitry for receiving a second received signal having a same frequency as a carrier frequency of the received modulated carrier signal and for producing at its outputs signals for controlling operation of the sample and hold circuitry. The sample and hold demodulator circuit provides a single IC solution, allowing amplitude and phase demodulation to be performed with a single sample and hold circuit.